In the audio field, transmitting audio signals using radio waves has been put to practical use because connecting a plurality of apparatuses using cables is burdensome or because cables are visually undesirable (for example, refer to JP-A-2006-074374). Frequency bands that can be used freely by consumer-oriented apparatuses for transmitting audio signals, for example, are limited, and not only apparatuses for transmitting audio signals but also various kinds of apparatuses, such as a wireless LAN, a cordless phone and a microwave oven, use a 2.4 GHz band.